The present invention relates to a developing device applicable to an electrophotographic copier, facsimile machine, printer or similar image forming equipment and, more particularly, to a vertical developing device advantageously applicable to a color copier.
A vertical developing device for the above application usually has a housing formed with an opening which faces an image carrier in the form of a photoconductive element. A developing sleeve is disposed in the housing and provided with a magnet therein. The sleeve is partly exposed to the outside via the opening of the housing. A sponge roller supplies a toner to a toner supply roller which, in turn, supplies it to the developing sleeve. An agitator agitate a fresh toner fed from a toner tank by a screw. A blade is held in contact with the toner supply roller to form a thin toner layer thereon. A trough-like screw casing is formed with a developer inlet and a developer outlet. An upper transport screw is surrounded by the screw casing and rotated in a predetermined direction for conveying the developer to the developer outlet. A lower transport screw is located above the developing sleeve for conveying the developer in a predetermined direction. A doctor is held in contact with the developing sleeve and forms a part of the housing. The housing is divided into a developer agitating chamber accommodating the sleeve, screws and doctor, and a toner storing chamber accommodating the agitator, blade, and rollers.
The developing device having the above construction allows the toner supply roller to supply a sufficiently charged fresh toner to the developer deposited on the developing sleeve. Hence, the interval between the time when the toner is supplied and the time when it is actually used for development is short, enhancing rapid response in toner supply control. In addition, since the frictional charging of the toner scarcely relies on the developer, the total amount of developer required to exist in the device can be reduced.
However, the conventional developing device has some problems left unsolved, as follows. Since the developer is not scraped off from the developing sleeve over the entire longitudinal area, the developer is continuously circulated due to the rotation of the sleeve. In this condition, the developer deteriorates rapidly, degrading the image quality. Moreover, the developer being continuously circulated due to the rotation of the sleeve cannot be fully agitated or mixed with the fresh toner fed from the toner supply roller. This brings about a change in density among individual copies, irregularity in density corresponding to the spiral pitch of each of the upper and lower screws, irregularity in density in the front-and-rear direction (axial direction of the sleeve) ascribable to defective lateral agitation, etc. Such changes and irregularities in density critically lower the image density in the event of, among others, full color development.
Further, in the conventional device, the agitation and mixture of the developer relies on the transporting force of the developing sleeve. Therefore, a decrease in the linear velocity of the sleeve translates into the short agitation of the developer. For this reason, the linear velocity of the sleeve cannot be made as low as the linear velocity of the image carrier.
The blade is held in contact with the toner supply roller to form a thin toner layer thereon, as stated earlier. The toner regulating ability of this roller decreases due to aging, i.e., changes in the surface roughness and other surface properties thereof ascribable to wear, toner filming, deterioration of rubber or similar surface material, etc.. Hence, the toner supply roller has to be cleaned or replaced from time to time. The blade also needs cleaning or replacement since the toner adheres thereto due to aging and lowers the toner regulating ability. So long as such members needing cleaning or replacement are positioned in the vicinity of the opening of the housing which faces the image carrier, they can be cleaned or replaced relatively easily. However, when such members are remote from the opening of the housing, they cannot be cleaned or replaced easily. Moreover, since these members form part of a partition delimiting the toner storing chamber, it is likely that the toner in the toner storing chamber is mixed with a two-component developer, i.e., a mixture of toner carrier existing in the developing and agitating chamber in the event of cleaning or replacement.